md description for intruction that modifies multiple operands

[Fred Fish]

> Note also that it is regarded as good style now do define constants in the 
> MD file for unspec operations.  Hence on ARM we now have

I've been putting that off since we need about 450 of them and
there have been quite a few changes since we mostly mechanically
generated all the templates.  Certainly before we submit our
changes to the baseline gcc we'll fix that.  :-)

-Fred